As is well known in the automobile industry, electric vehicles (EV) and hybrid-electric vehicles (HEV) may be provided with one or more high voltage batteries for powering the vehicle drivetrain. Such batteries require periodic re-charging after depletion, which may be accomplished by connecting the vehicle to a high voltage AC power line, which may be 120 or 240 volt AC, supplied by an electric utility grid. Such a connection may utilize a suitable vehicle connector configured to interface with a vehicle on-board battery charger (OBC). The OBC converts the grid AC power into appropriate DC power to charge the high voltage batteries. In order to perform the basic power conversions, all circuit components need to be powered by individual supplies in each isolated ground zone. This is done by Housekeeping Power Supplies (HKPS).
Electric and hybrid-electric vehicles may also include a low voltage battery, such as a 12 volt DC battery, for powering low voltage vehicle electrical systems and circuitry. Some or all of such electrical systems and/or circuitry of an EV or HEV could be supplied with energy from a high voltage power source when the vehicle is connected to an AC power line. In that event, such a power source is virtually unlimited and such an arrangement would help preserve the charge and/or extend the life of the vehicle 12 volt DC battery. In the absence of the vehicle connection to the AC power line, certain control circuitry could be supplied with sufficient power from the vehicle 12 volt DC battery to perform various diagnostic functions and/or to permit reprogramming of that control circuitry, such as re-flashing for software updates.
Such an arrangement typically requires two flyback transformers to individually regulate the AC power and the 12 volt battery power. The two transformers will never operate at the same time, resulting in an inefficient usage of materials and printed circuit board (PCB) space. Moreover, it requires seamlessly switching between AC power and the vehicle 12 volt DC battery, depending upon the presence or absence of the vehicle connection to the high voltage AC power line. Such switching may be accomplished using an auxiliary flyback converter to convert 12 volt DC to resemble the AC power at the minimum voltage level. When the AC power line comes alive, the flyback converter feedback will saturate and the converter will enter into a low power mode. Once the AC power is removed, the feedback will go out of saturation and the converter will resume its normal operation mode. The use of such an auxiliary flyback converter is bulky and costly, inefficient because all parts are running, and less reliable since both sides are tied together leading to a possible failure that can cause excessive high voltage. Other solutions include two separate flyback converters with shared outputs. A system microcontroller is employed to detect the state of AC power and then enable/disable one of the two converters. However, the microcontroller timing becomes critical and cannot meet the requirement for seamless transitions in most of the cases.